Plate-fin tube type of heat exchangers 3 comprising aluminum fins 1 and heat exchanger tube 2 as shown in FIG. 14 have been widely used as a heat pump type of air-conditioning and heating apparatus and so forth. A fluorinated hydrocarbon type of refrigerant such as R-22, R-11 and so on flows through the tube 2 to carry out heat exchanging operation with air passing between the fins 1. In such heat pump type of air-conditioning and heating apparatus, a single heat exchanger 3 functions as a condensor for heating operation in winter and also as an evaporator for cooling operation in summer. This means that the tube 2 is subjected to heat transfer with condensation in winter and heat transfer with evaporation in summer.
There has been known a method for preparing a heat exchanger tube having a porous layer formed by aluminum type sintered metal plate as disclosed in Japanese Examined Patent Publication No. 23065/1986 in order to improve evaporating heat transfer characteristics in the conventional heat exchanger tube 2. According to such method, the porous layer plate made of aluminum type sintered metal is metallically bonded on the wall surface of the tube 2 through alloying bonding material as shown in FIG. 15 to form the porous layer 4 on the entire wall surface of the tube 2. An evaporated refrigerant is captured in cavities formed in the porous layer 4 to work as bubble nuclei so as to accelerate the generation of bubbles. That helps excellent heat transfer characteristics to be obtained. With respect to "Nucleate Pool Boiling Heat Transfer from Porous Heating Surface", "Transactions of the Japanese Society of Mechanical Engineering (Part B) vol. No. 50, 451 (1984-3)", page 818, describes that the porous layer 4 is formed by bonding spherical metal particles having uniform particle size on the entire plain or smooth wall surface of the heat exchanger tube by means of electroplated film so as to obtain excellent bubble nuclei boiling heat transfer characteristics.
On the other hand, there have been known two methods for improving condensing heat transfer characteristics in the tube 2. One is a method for increasing heat transfer area by forming grooves 5 in the inner wall surface 2a of the tube 2 as shown in FIG. 16. The other is a method for improving condensing heat transfer characteristics by coiling a single steel wire 6 on and around the entire outer wall surface 2b of the tube 2 for heat transfer with condensation as shown in FIG. 17 (see page 2436 of "Transactions of the Japanese Society of Mechanical Engineering (Part B) vol. 51 No. 467 (1985-7)").
A heat exchanger tube utilized in the heat pump type of air-conditioning and heat apparatus is required to improve both evaporating heat transfer characteristics and condensing heat transfer characteristics.
When the tube 2 having the porous layer 4 as shown in FIG. 15 is utilized as a condensor, it is inferior to the tube 2 with the grooves 5 in its inner surface 2a as shown in FIG. 16 in terms of condensing heat transfer characteristics because condensate is held in the cavities in the porous layer 4 by capillary effect and is unapt to leave, and the liquid film functions as heat resistance. On the other hand, when the tube 2 with the grooves 5 as shown in FIG. 16 is utilized as an evaporator, it is quite inferior to the tube 2 with the porous layer 4 as shown in FIG. 15 in terms of evaporating heat transfer characteristics, though it is possible to improve the evaporating heat transfer characteristics in respect of the increment of the heat transfer area. It has a disadvantage that it can not improve both evaporating heat transfer characteristics and condensing heat transfer characteristics.